This invention relates to an aqueous electroplating solution for high speed electroplating satin bright tin-copper solder coatings.
Electroplated solder coatings are used extensively in the electronics industry in the manufacture of printed wiring boards (PWBs), electrical contacts and connectors, semiconductor packaging, electrical conduits and other related parts. These plated solder coatings must be pore free, corrosion resistant, display long term solderability and be free from solderability failures such as nonwetting and dewetting.
Until recently, lead and lead-tin alloys have been the most widely used solder coatings. But because of the toxicity of lead there has been a world-wide drive to replace lead solders in electronic parts. As a consequence there is an ongoing effort to find a replacement for tin-lead solder that can be xe2x80x9cdroppedxe2x80x9d into current applications. Electrodeposited tin-copper alloys are possible alternatives to commonly used tin-lead solder coatings.
Electroplating chemistries for tin-copper alloy plating have been known for many years. However, they were mainly used for electroplating of bronze, which is a tin-copper alloy with copper content normally between 20 and 50%. Such chemistries can not be utilized for solder plating with low copper content as required by the electronic industry. With the recent impetus to eliminate lead, new chemistries were developed for tin-copper solder plating. See, for example, I. Yanada et al., Eur. Pat. 1001054. These plating solutions, however, suffer from three major deficiencies: 1) they give a matte appearance, 2) they operate at relatively low current densities (up to 50 ASF) and 3) they have problems with copper immersion deposition. In order to overcome the matte appearance of the electrodeposited tin-copper alloy, a bright process was developed. However, the bright deposits often fail solderability tests.
Accordingly there is a need for a immersion-free electroplating solution that can produce high speed, satin bright plating of tin-copper alloy that will meet the solderability requirements for PWB and electronics packaging applications.
In accordance with the present invention, an electroplating solution is provided for electroplating satin bright tin-copper alloy solder coatings at high speed. The preferred solution comprises sulfonic acid, tin sulfonate, copper sulfonate, non-ionic surfactant, satin brightener and an antioxidant catechol. The preferred surfactant is polyoxyethylene-block-polyoxypropylene. The preferred satin brightener is formed by the oxidation of an aqueous solution of 1-phenyl-3-parazolidinone. The preferred sulfonic acid is methanesulfonic acid.